1. Field of the Invention
The present invention relates to radio frequency (RF) transponders, and more particularly, to a radio frequency identification (RFID) transponder that can preserve state information after losing power for a short period of time.
2. Description of Related Art
In the automatic data identification industry, the use of RFID transponders (also known as RFID tags) has grown in prominence as a way to track data regarding an object to which the RFID transponder is affixed. An RFID tag generally includes a semiconductor memory in which digital information may be stored, such as an electrically erasable, programmable read-only memory (EEPROM) or similar electronic memory device. An RFID interrogator or reader may recover the digital information stored in the RFID tag using modulated radio frequency (RF) signals. One such communication technique is referred to as xe2x80x9cbackscatter modulation,xe2x80x9d by which an RFID tag transmits stored data by reflecting varying amounts of an electromagnetic field provided by the RFID interrogator by modulating the antenna matching impedance of the tag. The RFID tag can therefore operate independently of the frequency of the energizing field, and as a result, the interrogator may operate at multiple frequencies so as to avoid RF interference, such as utilizing frequency hopping spread spectrum modulation techniques. Since RFID tags using backscatter modulation do not include a radio transceiver, they can be manufactured in very small, lightweight and hence inexpensive units.
RFID tags either extract their power from the electromagnetic field provided by the interrogator (also known as field-powered or xe2x80x9cpassivexe2x80x9d RFID tags), or include their own internal power source (e.g., battery). Passive RFID tags that extract their power from the interrogating field are particularly cost effective since they lack a power source, and can be constructed in smaller package sizes. A drawback of passive RFID tags is that they are susceptible to temporary fluctuations in power level due to variations in the RF environment. More particularly, RFID tags are often utilized in a physical environment that contains various RF absorbing and reflecting surfaces, such as within a manufacturing facility or warehouse. An RFID interrogator may be utilized within such a location to interrogate all of the RFID tags present within the location. The RF absorbing and reflecting surfaces of the location cause multipath cancellation, i.e., the complete cancellation of signals due to the relative amplitude and phase differences of RF components traveling over separate paths. This multipath cancellation is further compounded by the use of a frequency hopping spread-spectrum RF field pattern emitted by the RFID interrogator. As a result of the multipath cancellation, there may be areas within the location in which the RF field strength is essentially zero. Thus, passive RFID tags disposed within or passing through these zero field strength areas will temporarily lose power. In applications in which the RFID tag is expected to maintain its state after it is powered, the temporary loss of power destroys the state information held by the RFID tag.
For example, certain RFID systems include a command set that enables an RFID interrogator to execute a number of functions on plural RFID tags within its range. Using certain commands within the command set, the RFID interrogator may be able to identify multiple RFID tags simultaneously, or may be able to select a subset of RFID tags based on tag memory contents. These RFID tags may further include a state machine that undergoes transitions in the course of processing a command received from the RFID interrogator. When such an RFID tag momentarily loses power due to being within or passing through a zero field strength area, the internal information defining the state is lost. After the RFID tag power is restored, the tag may reinitialize in a state that is different than the state it was in prior to the loss of power. The RFID interrogator will have to repeat the transmission of commands to the RFID tag in order to restore the lost state and complete the desired transaction. This redundant transmission of commands results in an undesirable delay in communication between the RFID interrogator and tag. This communication delay particularly impacts RFID system protocols that identify the presence of multiple RFID tags within an environment, since the delay greatly increases the amount of time necessary to fully identify all of the tags.
In these and other RFID applications, it is very desirable to reduce the amount of time for accomplishing an identification transaction (and thereby increase the identification rate). It would therefore be advantageous to provide an RFID tag that can preserve state information after losing power for a short period of time.
The present invention provides an RFID transponder that includes a state holding cell that maintains the present state of the RFID transponder during temporary losses of power. After power is restored to the RFID transponder, the state holding cell restores the present state to the RFID transponder so that transactions with an RFID interrogator can continue without having to re-transmit redundant commands.
More particularly, the RFID transponder comprises an RF front end adapted to receive an interrogating RF signal. An analog circuit is coupled to the RF front end and is adapted to recover analog signals from the received interrogating RF signal. The analog circuit provides state information defining a desired state of the RFID transponder corresponding to the analog signals. A digital state machine is coupled to the analog circuit and adapted to execute at least one command in accordance with the state information. A memory is coupled to the digital state machine and is adapted to store and retrieve digital data responsive to at least one command executed by the digital state machine. A power capacitor is coupled to the RF front end and derives a voltage rectified from the interrogating RF signal to charge the power capacitor. In an alternative embodiment, the power capacitor derives the aforementioned rectified voltage from the interrogating RF signal via the analog circuit. Within such alternative embodiment, it should be appreciated that the power capacitor is not connected to the RF front end and is instead coupled between the state holding cell and the analog circuit. In either embodiment, however, the power capacitor provides electrical power for the analog circuit, the digital state machine and the memory. The state holding cell is coupled to the analog circuit and the digital state machine and is adapted to maintain the state information during a loss in power provided by the power capacitor due to lapse in receipt of the interrogating RF signal by the RF front end.
In an embodiment of the invention, the state holding cell further comprises an OR gate have a first input terminal operatively coupled to the analog circuit to receive a voltage corresponding to the state information, a second input terminal coupled to a capacitor, and an output terminal providing the state information to the digital state machine. The voltage charges the capacitor. A diode is coupled between the first input terminal and the second input terminal of the OR gate. A latch may also be coupled between the first input terminal and the output terminal of the OR gate. The latch is operative to restore the voltage corresponding to the state information to the first input terminal following the temporary lapse in receipt of the interrogating RF signal.
A more complete understanding of the passive RFID tag that retains state after temporary loss of power will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings which will first be described briefly.